In the next-generation power grid called as a smart grid, a smart meter (hereinafter, referred to as an SM) that collects power usage amounts of electrical devices is installed for each counting range of the power usage amounts such as each house. The SM communicates with a meter data management system (hereinafter, referred to as an MDMS), which is a data management apparatus, through the power grid. The MDMS collects a power usage amount for every unitary time from the SM disposed in each house or the like. The power usage amounts for every unitary time, which are collected by the MDMS, for example, are used by an energy management system (hereinafter, referred to as an EMS) connected to the power grid. The EMS performs power control such as requesting each house and the like within a management target area to suppress the use of power or controlling charging/discharging a storage battery connected to the power grid based on a total amount of the power usage of a plurality of houses and the like collected in the MDMS. In addition, the power usage amounts collected by the MDMS are also used in an accounting server connected to the power grid. The accounting server performs an accounting process for the use of power in each house based on the total amount of the power usage amounts of each house collected by the MDMS for a predetermined period. In this manner, the EMS, the accounting server, and the like (hereinafter, collectively referred to as application servers) use a total amount of the power usage collected by the MDMS from SMs for executing a predetermined application. Accordingly, the MDMS maintains the power usage amount collected from each SM, so that a total amount of power usage, which is used thereafter by application servers, can be calculated.
However, in a case where a configuration is employed in which the power usage amount collected up by each SM is maintained by the MDMS, a supervisor of the MDMS or an unauthorized user penetrating into the MDMS sneaks to look at the power usage amount, and accordingly, the situation of activities and the like in each house can be inferred, which leads to the infringement on privacy. Accordingly, a method for protecting privacy is considered in which a total amount of power usage needed by application servers can be calculated while the power usage amount collected by each SM is maintained in the MDMS in a concealed state. In order to maintain the power usage amounts in the MDMS in the concealed state, it is effective to encrypt the power usage amount in each SM. However, in such a case, it is required not to incur an excessive load in each SM.